Command for adding a default constraint

sql

database-design

data-consistency

sql-best-practices

byAnton Shumikhin·Dec 31, 2024

To assign a default value to a column in SQL, use:

ALTER TABLE YourTable
ADD CONSTRAINT DF_DefaultName
DEFAULT 0 FOR ColumnA;

This command quickly creates a default constraint, DF_DefaultName in this case, for ColumnA in the YourTable thereby making unspecified entries default to 0.

Adding default constraints 101: Detailed explanation with examples

When database values are inserted without defining all column values, assigning default values to a column via SQL's DEFAULT constraint becomes vital for maintaining data consistency and preventing null values.

Creating a table with a default value:

CREATE TABLE Account (
    AccountID int,
    -- Surprisingly, some accounts start with 0 balance!
    AccountBalance decimal NOT NULL DEFAULT 0
);

Altering an existing table:

ALTER TABLE Account
-- Giving AccountBalance a little nudge (aka DEFAULT) to be 0
ADD CONSTRAINT DF_AccountBalance 
DEFAULT 0 FOR AccountBalance;

Naming: The game of default constraints

Explicitly-named DEFAULT constraints:

Enhance predictability and ease of reference, critical when you aim to alter or drop the constraint in the future.
Adhere to best practices for code maintenance and readability.

Implicitly-named DEFAULT constraints:

SQL Server auto-generates a system name like DF__TableName__Column__<random number>, leading to potential identification and management headaches.
It's a one-off solution when the constraint won't be directly referenced in future.

Situations when naming plays a vital role

Dropping a constraint by its name: When the time comes to eliminate a default value, knowing the constraint's name is paramount.
```
ALTER TABLE Account
-- DF_AccountBalance's time has come... to an end!
DROP CONSTRAINT DF_AccountBalance;
```
Implicitly-named constraints during migration: In database schema scripts for migration or version control, unnamed constraints generate different names each time leading to inconsistencies and potential deployment issues.